See if the freezer is frosted up, maybe not defrosting.If it's frosted up check the defrost timer (or control board) defrost thermostat and defrost heater.

See if the freezer fan is running.If the fan is not running check to make sure it's getting power. If it has power replace the fan.

Check to see if the compressor is running.

If the compressor is not running see if you have power to the start relay on the side of the compressor.If you have power to the start relay, but the compressor does not run ohm the compressor out.If the compressor ohms out ok then replace the start relay.